The present invention relates to a refrigerating apparatus in which a refrigerant circuit is constituted of compression means, a gas cooler, reducing means and an evaporator to obtain a supercritical pressure on a high pressure side.
The present invention also relates to a refrigerating apparatus comprising a first refrigerant flow and a second refrigerant flow regulated in accordance with predetermined control characteristics to obtain a supercritical pressure on a high pressure side.
Heretofore, in this type of refrigerating apparatus, a refrigerating cycle is constituted of the compression means, the gas cooler, the reducing means and the like, and a refrigerant compressed by the compression means releases heat in the gas cooler, has a pressure thereof reduced by the reducing means, and is then evaporated in the evaporator, to cool ambient air by the evaporation of the refrigerant at this time. In recent years, in this type of refrigerating apparatus, Freon-based refrigerant cannot be used owing to a natural environmental problem and the like. Therefore, an apparatus has been developed in which carbon dioxide as a natural refrigerant is used as an alternative of the Freon-based refrigerant. It is known that the carbon dioxide refrigerant has a very large difference between a high pressure and a low pressure, has a low critical pressure and is compressed to obtain a supercritical state on the high pressure side of the refrigerating cycle (e.g., see Japanese Patent Published No. 7-18602 (Patent Document 1)).
As to the above Freon refrigerant, a temperature of the refrigerant has a unique relation to a pressure thereof to perform a saturation cycle. On the other hand, in a supercritical cycle which obtains the supercritical state on the high pressure side as described above, one of the saturation cycle and a gas cycle is performed in accordance with an outdoor temperature. In the saturation cycle, the temperature of the refrigerant has a unique relation to the pressure thereof as in a case where the Freon refrigerant is used, but in the gas cycle, the refrigerant is not liquefied, which causes a problem that when the refrigerant in the refrigerant circuit becomes excess, the temperature of the evaporator lowers but a high pressure side pressure becomes abnormally high.
Moreover, in such a supercritical refrigerant cycle, on conditions that the temperature of the refrigerant at a gas cooler outlet rises owing to a cause such as a high heat source temperature on a gas cooler side (e.g., a high temperature of outside air which is a heat medium subjected to the heat exchange between the medium and the gas cooler), a specific enthalpy at an evaporator inlet increases, thereby causing a problem that a refrigerating effect remarkably deteriorates. In this case, to acquire a refrigerating ability, the high pressure side pressure needs to be raised, thereby increasing a compression power, to cause a disadvantage that a coefficient of performance also deteriorates.
Therefore, there has been suggested a so-called split cycle (two-stage compression one-stage expansion intermediate refrigerating cycle) refrigerating apparatus in which a refrigerant cooled by a gas cooler is branched into two refrigerant flows, one branched refrigerant flow (a first refrigerant flow) has a pressure thereof reduced by auxiliary reducing means and is then passed through one passage (a first flow path) of an intermediate heat exchanger, and the other refrigerant flow (a second refrigerant flow) is passed through the other flow path (a second flow path) disposed so as to perform heat exchange between the flow path and the first flow path of the intermediate heat exchanger, and is then evaporated by an evaporator via main reducing means.
In the above split cycle apparatus, the first refrigerant flow obtained by branching the refrigerant which has released heat in the gas cooler can have the pressure thereof reduced and be expanded to cool the second refrigerant flow, whereby the specific enthalpy at the evaporator inlet can be decreased. In consequence, a refrigerating effect can be improved, and a performance can be enhanced effectively as compared with a conventional apparatus. However, a cooling effect by the first refrigerant flow for cooling the second refrigerant flow before reducing the pressure of the second refrigerant flow depends on the amount of the first and second refrigerant flows passing through the intermediate heat exchanger.
That is, when the amount of the first refrigerant flow is excessively large, the amount of the second refrigerant flow finally evaporated by the evaporator becomes inadequate. Conversely, when the amount of the first refrigerant flow is excessively small, the cooling effect by the first refrigerant flow (i.e., the effect of the split cycle) diminishes. On the other hand, the pressure of the first refrigerant flow having the pressure thereof reduced by the auxiliary reducing means is the pressure of the refrigerant circuit on a medium pressure side, and control of this medium pressure side pressure requires control of the amount of the first refrigerant flow. Therefore, to obtain an optimum performance improvement effect, these refrigerant flows need to be appropriately controlled.
On the other hand, when the amount of the first refrigerant flow is so large that the flow cannot completely be evaporated and the flow returns to the compression means, a liquid backflow occurs in the second-stage compression means. Therefore, when a predetermined superheat degree is not kept in the intermediate heat exchanger, liquid compression by the compression means is incurred. Therefore, it is necessary to control the discharged gas temperature of the compression means while considering the efficiency of the refrigerating cycle, so that a larger superheat degree is acquired.
Moreover, in the above refrigerating apparatus, during a usual operation, the pressure of a medium pressure region of the refrigerant circuit (e.g., the medium pressure region of the refrigerant discharged from a first compression element of the compression means comprising two-stage compression means) is normally lower than the pressure of a high pressure region (the high pressure region of the refrigerant discharged from a second compression element of such compression means). On the other hand, at the start of the compression means, when the outdoor temperature is high, the compression means starts from a high pressure state of a low pressure region of the refrigerant circuit. Therefore, the pressure of the medium pressure region rises early. At this time, the refrigerant of the high pressure region of the refrigerant circuit is cooled by the gas cooler, and does not immediately reach a high temperature. Therefore, the pressure of the medium pressure region of the refrigerant circuit comes close to the pressure of the high pressure region, and differential pressure between the refrigerant pressure of the medium pressure region and the refrigerant pressure of the high pressure region cannot sufficiently be acquired, thereby bringing the compressed state of the refrigerant into a compression defect state.
This incurs the start defect of the compression means, and the cooling ability of the refrigerating apparatus remarkably deteriorates. Moreover, a problem might occur that the power of the compression means increases to deteriorate the efficiency of the refrigerating cycle.
On the other hand, usually when the compression means stops, pressures of the compression means on a discharge side and a suction side are equalized. Moreover, the compression means starts from an equalized pressure state. However, in the refrigerating apparatus which obtains the supercritical pressure on the high pressure side as described above, the refrigerant circuit on the high pressure side and the medium pressure side cannot sufficiently be sealed via a discharge valve in a sealed container constituting the compression means, and these pressures are easily equalized. On the other hand, the pressures of the refrigerant circuit on the low pressure side and medium pressure side are not easily equalized in the sealed container, partially because the container on the low pressure side is immersed into oil. Moreover, since there is a large pressure difference in the refrigerant circuit, predetermined time is necessary for equalizing the pressures of the whole refrigerant circuit, thereby causing a problem that start properties deteriorate.
Moreover, when the refrigerating apparatus is employed as a refrigerating equipment in a supermarket or the like, evaporators are arranged in parallel with each other in each showcase, and reducing means disposed for the evaporators are controlled to realize cooling control in each showcase. In each showcase, to eliminate frosting of the evaporators, a defrosting operation is executed. In this case, the reducing means of the evaporator to be subjected to the defrosting operation is closed, and the defrosting operation is performed by heating with a heater or the like, an off-cycle operation or supply of a hot gas through a hot gas pipe disposed in the showcase.
However, during the off-cycle operation, the defrosting operation requires much time. During the heating with the heater, steep rise of the number of components and steep rise of running cost thereof are incurred. Moreover, when the hot gas pipe is disposed, a problem occurs that the whole system becomes complicated.